Newt’s interactions with [the niffler] are based on Mr. Redmayne’s observations of a zoologist working with a baby anteater. “It would curl up into a little ball, and in order to make it relax, she would tickle his little belly. There’s a moment where the Niffler has gorged himself on glittery stuff, and Newt is trying to get him to release his jewels. So he ends up tickling him.”
It’s love at first sight. He sees her and he doesn’t know if it’s just the magic in the air or there’s something about her that’s just like they were meant to be together but they’re not allowed to be together.